A 75Se-labeled macromolecule formed by Clostridium sticklandii in the presence of antibiotics that inhibit either protein or DNA-dependent nucleic acid synthesis was shown to be amino acid transfer nucleic acid (tRNA). Stability and compositional studies suggest that 75Se in the labeled tRNA is located in the ring structure of a modified nucleoside. Incorporation of 75Se is not decreased in the presence of large molar excesses of sulfide of cysteine and therefore is highly specific for Se. The 75Se-labeled tRNA is formed at extremely low Se concentrations and does not turn over rapidly. Chromatographic analysis (see C.S. Chen's report) showed three readily separable 75Se-labeled tRNA species to be present. One of these is a prolyl-tRNA and the other two are in the process of being identified. The possible relationships of these selenium modified tRNAs to selenoprotein biosynthesis or to other regulatory processes will be studied.